GBI stands for “Ghostbusters International”, the franchise corporation under the auspices of Ghostbusters Inc.
Basically, it's what the original NY Ghostbusters turned the company into. Their national and international success demanded it. It's also how MTGB, BAGB, NOVA GB, JZGB, and the numerous other franchises have franchised.
Think of it like this…everytime you go into a “Subway” restaurant, that particular location is independantly owned and operated by local folks. But their location is a Subway franchise…the owner has paid the Subway home office big bucks and a seven percent franchise fee, to start a Subway restaurant in their area. In return, they get to use the “Subway” name and lease equipment, utilize recipes, etc.
As GBI franchises, we do the same thing. We pay a hefty fee up front to start the business, and a percentage of each month's profit goes to the GBI home office in NYC. In return, we get to use the GB logo and name, utilize patented GB technology, and have an exclusive territory in which to act as representitives of Ghostbusters International. Pretty cool huh?
As for the “Top Secret” GBI project, I haven't a clue man, sorry.